The present invention refers to aqueous solutions of reactive polymers suitable for the production of hydrogel materials upon irradiation with blue light. The hydrogels are especially useful for production-of intraocular lenses, which can be formed in-situ in the capsular bag in an eye having undergone surgical excision of a natural lens.
The natural lens of the human eye is a precisely formed structure of fibre cells containing about 65 percent water and 35 percent organic material, chiefly structural proteins. The proteins are responsible for the relatively high refractive index of 1.42 of the lens and are structured in such a way that there are negligible local variations in their density, resulting in a transparent lens. Aging or large stresses can change the morphology of the proteins causing a progressive loss of transparency. This is termed cataract formation and is irreversible and can eventually result in blindness.
Implantation of an intraocular lens (IOL) following cataract surgery is performed to replace the optical function of the natural lens. In order to remove the natural, cataractous lens, as well as to prepare for the introduction of the IOL, an incision is made into the eye. For many years most IOLs were made of poly(methylmethacrylate), a material with good optical characteristics and compatibility with the tissues of the eye. A disadvantage of PMMA is, however, that it is a very rigid material and the incision must be made large enough, at least 5-6 mm, for implantation of the IOL. With improved devices for less traumatic removal of the natural lens by phacoemulsification, requiring only a rather small, there was a need for lenses with deformable optics. In such small incision surgery an opening of only 3 mm, or less is required. Various silicone, acrylate and hydrogel lenses have been commercialized.
All incisions in the eye are accompanied by trauma, and so, although foldable lenses have been a great improvement, there is still a need for lenses than can be placed through an even smaller incision. A lens can be implanted through a 1.2 mm opening by injecting the lens material into the capsular bag as a fluid, followed by formation of a solid full size lens in-situ in the eye. An additional advantage of this technique is that, due to the formation of a fill sized lens, complications of conventional IOL implantation namely decentration and posterior capsular opacification may be overcome. Full sized lenses show excellent centration and there is evidence that they may prevent posterior opacification (PCO). Hydrogels are a class of materials that are very interesting for an injectable lens because they have the added advantage is that their aqueous composition approximates to that of the natural lens.
Hydrogels can be made by crosslinking aqueous polymer or monomer/crosslinker solutions. Since monomers are often toxic, the use of polymers is preferred for applications in the eye. Polymers to which a reactive group is attached, for example, an acrylate group, can be polymerized in the presence of water and form a hydrogel. This is a process of crosslinking polymer or prepolymer solutions have been described before. In U.S. Pat. No. 5,665,840 crosslinkable water-soluble prepolymers based on copolymers of vinylalcohol and vinyllactams are claimed for contact lens application. Several other publications also disclose the production of hydrogels after crosslinking crosslinkable water-soluble (pre)polymers. The crosslinkable water-soluble (pre)polymers are designed to polymerize in a mould yield hydrogel. However, in-situ in the eye, water is present which demands certain specific requirements of the crosslinkable water-soluble prepolymers not described in any of these publications.
It is evident that the coherence of the aqueous solution during injection and prior to crosslinking is very important. This coherence prevents, or limits, the water in the eye from interfering with the solution before crosslinking. Without it, the carefully regulated hydrogel composition, required for generating an IOL of precise refractive index, may be diluted or altered and it is also needed to prevent leakage of polymer from the capsular bag before the polymer is crosslinked.
In addition, all hydrogels have an equilibrium water content, which is controlled both by the structure of the hydrogel and its crosslink density. Water-soluble polymers, when crosslinked, have a tendency to swell in water. For this invention it is important that after injecting the aqueous hydrogel solution into the eye, and crosslinking it, the amount of water that it takes up afterwards, the additional swelling, is limited, that is to say that the solution concentration of water in the injected hydrogel solution is equal to, or very close to, the equilibrium water concentration of the crosslinked hydrogel formed in the eye. Any additional swelling will decrease the refractive index of the lens and in more severe cases will cause collapsing of the capsular bag or even damage the eye.
A few publications and patents describe the design of injectable and crosslinkable water-soluble (pre)polymers and some polymerizations, which form hydrogels in-situ in the eye.
Ravi et al. reported in Polymer Preprints, 1999, 40, 630 on an injectable hydrogel material made by thermally curing poly(ethylenglycol) monoacrylates and diacrylates (PEGMA and PEGDA, respectively). They, were, however, not studied for possible lens refilling material but as potential probes to study the accommodation mechanism. The polymer content and, therefore, the refractive indices of the materials were very low ( less than 1.36) and the materials showed severe swelling after curing.
In the International Patent Application WO 93/02639 an injectable collagen-based intraocular hydrogel lens is described. The concentration collagen was very low which resulted in a low refractive index of the intraocular lens (1.363). It was claimed that high refractive lenses could also be made. To achieve re-active indices closer to that of the natural lens much higher collagen concentrations are needed, than those reported in the patent, and this will increase the problem of swelling, post-injection.
In the International Patent Application WO 01/08604 it was claimed that with aqueous solutions of modified linear polymers, accommodative lenses could be prepared in-situ. The water-soluble polymers were modified to low degrees, to obtain lightly crosslinked hydrogels in order to keep the elasticity modulus of the hydrogel lenses low. The moduli of the hydrogels were measured directly after crosslinking the aqueous polymer solution. The hydrogel were, however, after crosslinking not subjected to an aqueous environment. If these materials would be made in the eye in the presence of an excess natural aqueous fluid, densities they may be expected to swell significantly due to their low crosslink density. Such swelling is to the detriment of the refractive indices of the lenses they form.
In the International Patent Application WO 00/47185, a method of producing an injectable hydrogel intraocular lens is described. The hydrogel lens materials are based on macromolecular particles. However, !the problems of obatining a suffcient coherence and avoiding additional swelling are not addressed in this disclosure.
It can be concluded that the specific requirements regarding coherence and prevention of additional swelling for injectable hydrogel lenses still needs attention if a clinically acceptable surgical process is to be attained.
An object of the present invention is to provide an aqueous composition of a water soluble linear polymer that can fill the capsular bag of the eye and upon a crosslinking reaction by means of irradiation with light in the visible range can form a hydrogel intraocular lens implant of a predetermined refractive index.
It also an object of the present invention to provide an aqueous composition of a linear water soluble which has suitable coherence to avoid leaking from the capsular bag or avoiding dispersion effects in the capsular bag, while having suitably low retention time before starting the lens forming light induced crosslinking reaction.
An another object of the invention is to sufficiently crosslink the lens so that additional swelling is prevented and in this way ensure that the implanted lens permanently retains its predetermined shape and refractive value.
It is a still further object of the invention to provide hydrogel lens with no or minimal extractable polymer material.
In its most general terms the present invention pertains to an aqueous composition of a of water soluble polymers having a sufficiently high coherency that it substantially not is dispersed when being injected into an aqueous environment to undergo a crosslinking reaction into a hydrogel. The water soluble polymers can either be a water-soluble polymer bearing free acrylic, or other vinylic groups capable of facile free radical reaction with a polymeric eater-soluble photoinitiator also present in the composition, or a water-soluble polymer bearing both free acrylic, or other vinylic groups capable of facile free radical reaction and photoinitiator groups. Both these polymers will be crosslinked when irradiated with light of wavelengths greater than about 305 nm induces the photoinitiator groups to form free radicals so as to form the hydrogel. Preferably, the aqueous composition shall form a clear hydrogel with a refractive index of between about 1.36 to about 145, a transmission of visible light of at least about 35% with no, or substantially no, absorption of water from an aqueous environment during its formation or subsequently thereafter, so it becomes dimensionally stable and substantially free from swelling after the forming crosslinking process. These features will render the aqueous compositions improved properties to form an intraocular lens, in-situ in the capsular bag of the eye, from which defect natural lens has been surgically excised.
It is an important aspect of the invention that the aqueous composition has a suitable coherence to enable a surgical procedure comprising injection into the capsular bag of the eye and crosslinking to form a lens implant replacing an excised natural crystalline lens. The coherence at zero shear stress of the aqueous composition shall be sufficiently high, so that the injected composition does not leak from the capsular bag and that the composition does not disperse when injected through a standard injection needle into an aqueous environment, such as the capsular bag of the eye. It is therefore preferred that the composition has a zero shear stress viscosity of at least 3.0 Pas, preferably 10.0 Pas, and most preferably greater than 30.0 Pas. At the same time, the coherence of the inventive aqueous composition at zero shear stress must not be too high, since this will lead to an inconveniently long relaxation time of the injected aqueous composition before it is sufficiently relaxed to assume precisely the interior contours of the capsular bag and is ready to undergo crosslinking into the lens implant. An excessively coherent aqueous composition will compromise the capacity of the capsular bag to act as a forming mould for the lens surface by exerting a backpressure, with the result of an uneven or bumpy surface of the final lens implant. The coherence of an aqueous solution is determined by the interactions between the solvated polymer molecules, and the stronger these interactions the higher the coherence. These interactions derive from some combination of the generally recognized intermolecular forces, van der Waals, polar, H-bonding, and London or dispersion forces, and for polymers an additional form of interaction, chain entanglement, is a significant contributor. Chain, entanglements are physical crosslinks, which occur in polymer solutions, where the polymer has a molecular weight above a context-determined threshold. The degree of chain entanglement is also concentration dependent. These various interactions combine in any given solution to determine that solution""s viscosity, and so a solution""s viscosity is a useful indicator of its coherence threshold.
In order to determine if the presently invented compositions exhibit a sufficient coherency, rheological tests similar to those provided in Carbohydrate Polymers, 2000, 47, 109-119 (GS Harding et al) can be followed. According to these to tests cohesive compositions of hyaluronic acid, such as Heaton(copyright) 5, have a distinguishable behavior when flowing in PTFE, tube that are desirable for the presently invented compositions.
In the present invention the interactions between the polymer chain is determined by the chemical nature of the modified polymer the concentration and the molecular weight. Generally, a polymer modified with a hydrophobic crosslinker, will have greater coherence for a given molecular weight and concentration than a polymer modified with a hydrophilic crosslinker, where a higher molecular weight or concentration is needed for the same coherence. It is a part of the present invention to select the polymer, its molecular weight, the functional groups for crosslinking and polymer concentration in such a way that the refractive index of the aqueous solution and the coherence are in a suitable range for an injectable solution that shall form a lens implant with a controlled and predetermined refractive value.
It is a prerequisite for the present invention that the aqueous composition shall have suitable refractive to be able to provide a lens with a refractive index similar to that of the natural crystalline lens, i.e. the composition shall have a refractive index in the range of about 1.36 to 1.45. For a given hydrophilic polymer, the refractive index of a hydrogel lens is dependent upon both the mole fraction of the polymer, and the mole fraction of water in the hydrogel and the refractive index of each. Experience has shown that, for example, when polyvinyl alcohol with a refractive index of 1.51 is used to form a hydrogel, a polymer concentration of 14 to 54 wt % is needed to obtain refractive indices of 1.36 to 1-45. For the linear, water soluble, polymers employed in the present invention, the aqueous concentration required to obtain any desired refractive index will exceed the critical concentration of about 5%. Above this concentration polymer coils start to overlap and the polymer solutions to attract water, to reduce the concentration. If the equilibrium concentration is not attained during injection swelling of the formed crosslinked gel, the lens, will result as it reverts to its equilibrium water content. According to the present invention, the tendency of swelling of the finally crosslinked lens is entirely or substantially eliminated by introducing a sufficiently high crosslink density, as is dependent by the amount of the introduced functional groups for crosslinking on the linear water soluble polymers and above exemplified with acrylate groups. The interaction parameter of the polymer and the crosslinker with water will determine the optimal crosslink density. When the polymer as well as the crosslinker have a relatively high interaction parameter with water, more crosslinks are needed to prevent additional swelling, compared to a polymer and crosslinker with a lower interaction parameter. This also counts for the interaction parameter of the polymer and crosslinker separately. If the same polymer is used, and a crosslinker with a high interaction parameter is used, the needed crosslink density is higher than in case of a crosslinker with a lower interaction parameter. With the same crosslinker, a polymer with a high interaction parameter needs more crosslinks than a polymer with a low; interaction parameter. The interaction parameters "khgr" of the polymer and cross linker will also determine the transparency of the hydrogel. When the interaction parameter of the polymer and crosslinkers are too different, this means that the two components are not miscible which will lead to phase separation and opacification of the hydrogel. For a good transmission the interaction parameters of the crosslinker and polymer should be in the same range. In a preferred embodiment hydrophilic water soluble polymers are copolymers of vinyl alcohol and have at least one monomer unit having a lactam group, such as vinyl pyrrolidone.
Further, the copolymers have at least one monomer unit capable of acting as a functional group in the crosslinking reaction, such as an acrylate group or a derivative thereof. Advantageously these monomer units are randomly distributed on the polymer chains and present in an amount yielding between about 15 and 20 mol % of acrylate groups to obtain a sufficiently high cross linking density. A complementing advantage of a highly crosslinked hydrogel according to the present invention, is that all the polymer chains are incorporated into the gel and so the system has no, or a minimum amount of extractable agents.
In one embodiment, the present; invention relates to a mixture of two water-soluble polymers of which one is a polymer having functional groups for crosslinking attached and one is a polymer having photoinitiator groups attached. The copolymers have a general formula (A)m(B)n(C)p(D)q and (A)m(B)n(D)q(E)r in which A can one of the following monomer, vinyl amides, vinyl lactams, acrylamide, methacrylamide, N-substitued acrylamides, N-substitued methacrylamides, vinylamine, ethyleneoxide, vinylsulfuric acids, vinylphosphonic acids, maleic acid, vinylpiperidine, vinylacrylate, methylvinylether, ethylene imine, methacrylic acid, acrylic acid, and vinylammonium salts. B is vinylalcohol or similar alcoholic group, C is a crosslinkable group attached to vinylalcohol or another alcoholic group, by its reaction with a suitable reagent such as acrylic acid chloride, methacrylic acid chloride, isocyanato acrylate, isocyanato methacrylate, epoxy acrylates, epoxy methacrylates, or itaconate or aconitate acid anhydride. D is optionally a monomer to modify the refractive index of the polymer, such as methylmethacrylate, styrene, or methyl- or benzyl-N-acetamidoacrylate, and E is a photoinitiator moiety carrying a photoinitiating group which is described per se in the International Patent Application WO 00/55214 which hereby is incorporated as a reference.
An alternative to this polymer mixture is a water-soluble copolymer to which both the functional group for crosslinking, and the photoinitiator group are attached to the and has the following general formula (A)m(B)n(C)p(D)q(E)r where A, B, C, D, E are equally as described above.
According to a preferred embodiment, the water soluble polymer has the general formula:
(A)m(B)n(C)p(D)q 
in which A, B, C and D are each are vinyl groups (xe2x80x94CH2xe2x80x94CHxe2x80x94) to which a pyrrolidone group (A), a hydroxyl or acetate group (B), an acrylate group (C), and a photoinitiator group (D), respectively, is attached; and where the respective mole fractions of the different monomer units is: m=0.1-0.5; n=0.1 to 0.5; p=0.1-0.2; and q=0.0-0.1. The photoinitaitor groups and the acrylate groups are preferably randomly distributed on the polymer chains. Most advantageously, all groups A, B, C and D are randomly distributed along the chains in order to avoid zones with different hydrophilic characteristics which at worst may lead to phase separation and a compromised optical quality.
The photoinitiator groups are preferably selected so as to be able to bring about a crosslinking reaction induced by irradiation with blue light at a wavelength of about 400 to 500 nm. To accomplish this performance, the photoinitiator groups suitably comprise a phosphine oxide moiety. Typically suitable monomers (D) are 2,6-dimethylvinylbenzoyldiphenylphospine oxide, preferably 2,6-dimethyl-4-vinylbenzoyldiphenylphosphine oxide that typically can be present in an mole fraction of amount in the range of about 0.05 to 0.
In a specific example of the aqueous composition the water soluble polymer can follow the formula (A)m(B)n(C)p(D)q, wherein A, B, C and D are each are vinyl groups (xe2x80x94CH2xe2x80x94CHxe2x80x94) to which a pyrrolido group (A), a hydroxyl or acetate group (B), an acrylate group (C), respectively is attached and where the respective mole fractions of the different monomer units are: m=0.1-0.5; n=0.1-0.5; p=0.1-0.2; and q=0. The photoinitiator is present as a separate water soluble polymer entity, which is a copolymer of 2,6-dimethyl-4-vinylbenzoyldiphenylphosphine oxide and N,N-dimethylacrylamide with 2,6-dimethyl-4-vinylbenzoyldiphenylphosphine oxide present in the water soluble photoinitiator in mole fractions in the range of 0.05 to 0.25. Alternatively, the photoinitiator is present as a separate water soluble polymer having the formula (A)m(B)n(D)q, wherein A, B and D are each are vinyl groups (xe2x80x94CH2xe2x80x94CHxe2x80x94) to which a pyrrolidone group (A), a hydroxyl or acetate group (B), and a photoinitiator group (D), respectively, is attached; and where the respective mole fractions of the different monomer units is: m=0.1 to 0.5; n=0.1 to 0.5 and q=0.05-0.25. In the photoinitaitor group (D) the phosphine oxide can be linked to the vinyl groups through a urethane band as outlined in detail in the above mentioned WO 00/55214. In this specific example of the invention the water soluble polymer to be reacted with the polymeric photoinitiator suitably is a copolymer of vinyl pyrrolidone and vinyl alcohol wherein a fraction of the hydroxyl groups modified with a functional acryl group for crosslinking. Typically about 15 to 20 mol % of the hydroxyls are modified in order to obtain the suitable crosslinking density as earlier discussed.
The aqueous compositions so far disclosed typically will find use in methods of forming a rigid and dimensionally stable hydrogel intraocular lens in situ in the eye by. The compositions will be injected with a conventional syringe into the capsular bag of the aphakic eye. Due to the suitable coherence and viscosity of the composition this can be performed without complications in the form high injection pressure, air bubble formation or material dispersion which otherwise may jeopardize patient safety or lens quality. Subsequent to injection and relaxation of the material the eye is irradiated with a sufficient amount of light exceeding about 305 nm, preferably with blue light to form the hydrogel lens
The present invention also relates a method of forming a dimensionally stable clear hydrogel that includes at first the provision of a vinyl alcohol monomer containing polymer having a refractive index of at least 1.41. A first part of the polymer is chemically modified so a sufficient amount of a first part of said vinyl alcohol monomers obtains functional acrylic groups for crosslinking. A second part of said vinyl alcohol containing polymer is chemically modified by attaching a blue light activated photoactive group comprising a phosphine oxide moiety, for example by means of an urethane bond, so as to obtain a polymeric water soluble photoinitaitor. The first and second part is mixed into an injectable aqueous composition with a sufficient polymer concentration to obtain a refractive index of about 1.36 to 1.45. Finally, the composition is injected into the eye or into another body site and irradiated with light of a wavelength exceeding about 400 nm, so as to form a hydrogel by means of crosslinking.
Besides preparation of intraocular lenses the inventive aqueous hydrogel foxing compositions will find use in numerous other medical applications including tissue engineering, tissue adhesive processes. The hydrogels may also act as slow release or depot formulations for a wide variety of pharmaceuticals as for example illustrated by, but not limited to agents that inhibit epithelial cell growth on an implanted intraocular lens.
The following detailed description aims to illustrate examples of hydrogels according to present invention and should not be regarded as limiting for the scope of the invention or its applicability.